The study of clouds, where they occur, and their characteristics, play
a key role in the understanding of climate change. Low, thick clouds primarily
reflect solar radiation and cool the surface of the Earth. High, thin clouds
primarily transmit incoming solar radiation; at the same time, they trap
some of the outgoing infrared radiation emitted by the Earth and radiate
it back downward, thereby warming the surface of the Earth. Whether a given
cloud will heat or cool the surface depends on several factors, including
the cloud's altitude, its size, and the make-up of the particles that form
the cloud. The balance between the cooling and warming actions of clouds
is very close although, overall, averaging the effects of all the clouds
around the globe, cooling predominates.

The Earth's climate system constantly adjusts in a way that tends toward
maintaining a balance between the energy that reaches the Earth from the
sun and the energy that goes from Earth back out to space. Scientists refer
to this as Earth's "radiation budget." The components of the Earth
system that are important to the radiation budget are the planet's surface,
atmosphere, and clouds. The energy coming from the sun to the Earth's surface
is called solar energy. Most of it is in the form of radiation from the
"visible" wavelengths, i.e., those responsible for the light detected
by our eyes. Visible radiation and radiation with shorter wavelengths, such
as ultraviolet radiation are labeled "shortwave." Both the amount
of energy and the wavelengths at which energy is emitted by any system are
controlled by the average temperature of the system's radiating surfaces,
plus the emission properties. The temperature of the sun's radiating surface,
or photosphere, is more than 5500°C (9900°F). However, not all of
the sun's energy comes to Earth. The sun's energy is emitted in all directions,
with only a small fraction being in the direction of the Earth.

Energy goes back to space from the Earth system in two ways: reflection
and emission. Part of the solar energy that comes to Earth is reflected
back out to space in the same, short wavelengths in which it came to Earth.
The fraction of solar energy that is reflected back to space is called the
albedo. Different parts of the Earth have different albedos. For example,
ocean surfaces and rain forests have low albedos, which means that they
reflect only a small portion of the sun's energy. Deserts, ice, and clouds,
however, have high albedos; they reflect a large portion of the sun's energy.
Over the whole surface of the Earth, about 30 percent of incoming solar
energy is reflected back to space. Because a cloud usually has a higher
albedo than the surface beneath it, the cloud reflects more shortwave radiation
back to space than the surface would in the absence of the cloud, thus leaving
less solar energy available to heat the surface and atmosphere. Hence, this
"cloud albedo forcing," taken by itself, tends to cause a cooling
or "negative forcing" of the Earth's climate.

Another part of the energy going to space from the Earth is the electromagnetic
radiation emitted by the Earth. The solar radiation absorbed by the Earth
causes the planet to heat up until it is emitting as much energy back into
space as it absorbs from the sun. Because the Earth is absorbing only a
tiny fraction of the sun's energy, it remains cooler than the sun, and therefore
emits much less radiation. Most of this emitted radiation is at longer wavelengths
than solar radiation. Unlike solar radiation, which is mostly at wavelengths
visible to the human eye, the Earth's longwave radiation is mostly at infrared
wavelengths, which are invisible to the human eye. When a cloud absorbs
longwave radiation emitted by the Earth's surface, the cloud reemits a portion
of the energy to outer space and a portion back toward the surface. The
intensity of the emission from a cloud varies directly as its temperature
and also depends upon several other factors, such as the cloud's thickness
and the makeup of the particles that form the cloud. The top of the cloud
is usually colder than the Earth's surface. Hence, if a cloud is introduced
into a previously clear sky, the cold cloud top will reduce the longwave
emission to space, and (disregarding the cloud albedo forcing for the moment)
energy will be trapped beneath the cloud top. This trapped energy will increase
the temperature of the Earth's surface and atmosphere until the longwave
emission to space once again balances the incoming absorbed shortwave radiation.
This process is called "cloud greenhouse forcing" and, taken by
itself, tends to cause a heating or "positive forcing" of the
Earth's climate. Usually, the higher a cloud is in the atmosphere, the colder
is its upper surface and the greater is its cloud greenhouse forcing.

If the Earth had no atmosphere, a surface temperature far below freezing
would produce enough emitted radiation to balance the absorbed solar energy.
But the atmosphere warms the planet and makes Earth more livable. Clear
air is largely transparent to incoming shortwave solar radiation and, hence,
transmits it to the Earth's surface. However, a significant fraction of
the longwave radiation emitted by the surface is absorbed by trace gases
in the air. This heats the air and causes it to radiate energy both out
to space and back toward the Earth's surface. The energy emitted back to
the surface causes it to heat up more, which then results in greater emission
from the surface. This heating effect of air on the surface, called the
atmospheric greenhouse effect, is due mainly to water vapor in the air,
but also is enhanced by carbon dioxide, methane, and other infrared-absorbing
trace gases.

In addition to the warming effect of clear air, clouds in the atmosphere
help to moderate the Earth's temperature. The balance of the opposing cloud
albedo and cloud greenhouse forcings determines whether a certain cloud
type will add to the air's natural warming of the Earth's surface or produce
a cooling effect. As explained below, the high thin cirrus clouds tend to
enhance the heating effect, and low thick stratocumulus clouds have the
opposite effect, while deep convective clouds are neutral. The overall effect
of all clouds together is that the Earth's surface is cooler than it would
be if the atmosphere had no clouds.